1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having an intermediate flow mixing nonsupport grid at one or more strategic axial locations being tailored for optimizing the relative critical heat flux (CHF) performance of the fuel rod bundle.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In a typical boiling water reactor (BWR) fuel assembly, a bundle of fuel rods are subdivided into four separate mini-bundles by a central water cross and each mini-bundle is supported in laterally spaced-apart relation by a plurality of axially displaced grids or spacers, for example six in number, axially spaced apart along its fuel rods. The spacers are needed to maintain the desired fuel mini-bundle configuration. Then, all four mini-bundles of the fuel assembly are encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces such BWR fuel assembly is illustrated and described in U.S. Pat. No. 4,560,532 to Barry et al.
The critical heat flux (CHF) performance of a BWR fuel assembly is of paramount importance in the safe operation of a BWR. Upon occurrence of a CHF condition, the surface heat transfer coefficient drops by a large amount, leading to an exponential rise in fuel rod cladding temperature. This can cause rupture and release of fission fragments into the coolant. In order to avoid this condition, regulatory guidelines restrict BWR operation to a low enough level to avoid the CHF condition.
With respect to a BWR fuel assembly like that of the aforementioned patent, it has been realized that CHF performance is highest for fuel rods in the corner and side, or perimeter, spacer locations and lowest for fuel rods in the interior locations. This is due primarily to the hotter steam of the two-phase coolant tending to drift towards the interior fuel rod locations while the cooler liquid of the two-phase coolant accumulates at the channel along the perimeter fuel rod locations. As a result, premature dryout and degradation of CHF performance occurs at the interior fuel rod locations.
Consequently, a need exists to improve CHF performance at the interior fuel rod locations so as to enhance the overall CHF performance of the fuel assembly. While turbulence generating mixing vanes have been employed heretofore on PWR fuel assembly spacers to achieve mixing and homogenization of flowing coolant, for example as disclosed in the above cross-referenced applications and in U.S. Pat. Nos. 3,379,619 (to Andrews et al) and 3,395,077 (to Tong et al) being assigned to the assignee of the present invention, none of the designs disclosed are viewed as particularly adapted to provide the improvement being sought herein.